Biased-gap klystron



Sept. 20, 1966 1. EL-HEI-NI BIASEDGAP KLYSTRON 2 Sheets-Sheet 1 FiledAug. 1, 1965 R. F. SOURCE MAGNETIC CO MAGNETIC COIL FIG. I

INVENTOR.

IBR AIIINI EL HEFNI ATTORNEY Sept. 20, 1966 l. EL-HEFNI 3,274,430

BIASED-GAP KLYSTRON Filed Aug. 1, 1963 2 Sheets-Sheet 2 A o o o v S 6 gE o a Lt CC ul 0 l l I l I l O l 2 3 4 5 6 O I 2 V uw) VB (KV) FIG. 2FIG. 3

EFFICIENCY 0) V+V (KV) INVENTOR 4 IBRAHIM ELHEFNI ATTORNEY United StatesPatent 3,274,430 BIASED-GAP KLYSTRQN Ibrahim Ell-Hefni, North Andovcr,Mass, assignor to Massachusetts Institute of Technology, Cambridge,Mass, a corporation of Massachusetts Filed Aug. 1, 1963, Ser. No.299,350 7 Claims. (Cl. 315-5.41)

This invention relates to an electron beam microwave tube and inparticular to a multi-cavity klystron wherein an accelerating DC.voltage is applied along the RF. interaction region and morespecifically across the interaction gaps of the cavities.

In general, klystron tubes designed for high-power operation operate atrelatively low efiiciencies compared to other types of high frequencytubes, but are much more stable at high gains (30 db), and for thisreason continue to be preferred in certain applications despite lowerefficiency. However, even a slight increase in efficiency of thehigh-power klystron is highly desirable and much effort to provide thisincrease has been expended without great success. Moreover, thehighpower klystron has other operating characteristics which limit itspower capability. Among these are the problems of beam interception andmultipactor. It is to obtain improvement in all these operatingcharacteristics but especially efficiency that the apparatus of thisinvention is directed.

In the conventional multi-cavity klystron the electron beam isaccelerated to its full potential in the gun region before it isinjected into the RF. structure of the tube. In the klystron accordingto this invention, the beam is only partially accelerated in the gunregion and as it is being bunched by the radio frequency gap fieldsfurther D.C. acceleration is applied by one or more of these gaps. Ithas been found that the application of an accelerating D.C. potentialacross the gap of a cavity of a klystron produces an increase ofefiiciency, an increase in power output, an increase in stability and animprovement in beam transmission. One would expect, therefore, that themultipactor problem would be reduced. The extent of the influence uponany one of these items being dependent upon the particular cavity in themulti-cavity klystron to which the potential is applied. It has alsobeen found that additional improvement is obtained when the collector isat a lower potential than the accelerating DC. potential.

It is, therefore, an object of this invention to provide an improvedtube of the type employing R.F. cavities to provide velocity modulationof an electron beam.

It is another object to provide an improved klystron in which one ormore of its RF. cavity gaps has a direct voltage accelerating field.

It is a further object to provide a klystron in which the collector isat a lower potential than the accelerating potential.

The novel features which are believed to be characteristic of theinvention together with further objects and advantages thereof, will bebetter understood from the following description considered inconnection with the accompanying drawings in which several embodimentsof the invention are illustrated by way of example.

FIGURE 1 is a diagrammatic sectional View of an embodiment of theinvention together with associated circuitry.

FIGURE 2 is an illustrative diagram of the power output for a biasedoutput gap.

FIGURE 3 and FIGURE 4 are illustrative diagrams of efficiency versusbias voltage on various cavity gaps.

Referring now to the drawings, FIGURE 1 illustrates an embodiment of thepresent invention comprising a four-cavity klystron tube with three ofthe cavities direct ice voltage insulated from their associateddrift-tubes by means of RF. chokes. For clarity of illustration, inorder to more effectively present the novel features of this invention,the mechanical details by which adjacent drifttube sections areconnected to provide rigidity and a vacuum tight enclosure are notshown. Also, the supporting housing for the cathode 6 and the collector10 are omitted for a like reason.

Input cavity 1 is in direct electrical contact with the internaldrift-tube sections 2 and 3 to form an RF. resonant chamber as in aconventional klystron. This cavity is at direct ground potential. Whenthe klystron is operated as an amplifier, R.F. energy is induced incavity 1 by excitation of coupling loop 4 from a low-power RF. source 5.The cathode 6 heated by filament 7 are both electrically connected to anegative high voltage source V The potential V which exists betweencathode 6 and drift-tube section 2 causes the electrons emitted from thesurface of cathde 6 to be accelerated down through drift-tube sections 2and 3 in the form of a beam 8. The beam proceeds down the length of theklystron through drift tubes 12, 16, and 20 under the influence of thefocussing magnetic field of magnetic coils 9 until the beam 8 strikesthe collector 10.

The experimental evaluation of this invention was performed chiefly on acommercially available tube, the Eimac 4KM3000 LR, rated at 2 kw. as aC.W. amplifier at 8 kv. cathode voltage, having an electrically isolatedcollector, a narrow gap (0.8 radian), and designed for operation withfour external cavities. The efiiciency of this tube is relatively high(40 to 55% depending upon the accelerating voltage V when operated inthe conventional manner with all the cavities directly connected to thedrift-tube sections without gap bias. Some data was taken on a wide-gaptube (2 radians) for comparison purposes. As a general rule, the datagiven in this application which shows the superiority of biasedgapoperation on the narrow-gap klystron applies equally well and more so tothe wide-gap klystron.

Drift-tube 3 is shared by cavity 1 and the next buncher cavity .11 seenby the beam 8 as it progresses down the length of the klystron. Thecavity 11 is electrical-lyconnected to drift-tube 12 through R.F. choke13. Drifttube 12 is also electrically connected to the adjacent bunchercavity 14 through choke 15. Therefore, a DC. potential V may be directlyapplied to drift-tube 12 While allowing cavities 11 and 14 to be atground potential which is usually desired for safety and convenience.Cavity 14 is also electrically connected to drift-tube 16 through choke17. Drift-tube 16 is also connected through RF. choke 19 to the outputcavity 18 which is in turn connected to the final drift-tube 20 throughchoke 21. DC. potentials V and V may then be applied to the isolateddrift-tubes 16 and 20, respectively. It is to be noted that twodrift-tubes choke coupled to their associated cavity form a reentrantresonant cavity or resonator which when energized with radio frequencysignals produces an RF. voltage gradient in cavity gap. In addition, aDC. voltage gradient is produced across the gap by the difference involtage on adjacent drift-tubes. Therefore, gaps 22, 23, and 24 have RF.and D.C. voltage gradients if V V V 0. It should be noted that althoughFIGURE 1 shows drift-tubes 12, 16 and 20 at potentials V V and V it ispossible to operate this klystron wit-h drift-tube 12 grounded, oralternatively drift-tubes 12 and 16 both grounded. For example, if bothdrift-tubes 12 and 16 are grounded, cavities 14 and 18 may be directedconnected to these drift-tube sections as in a conventional klystronwithout the necessity of the RF. chokes 13, 15, 17 and 19. It

is also to be noted that since the potentials across gaps 22,

23 and 24 are desired to be accelerating potentials relative to themotion electron beam 8, it follows that the potentials V V V bemaintained at all times.

For biased output gap operation with undepressed collector, only theoutput cavity gap 22 is to be biased with an accelerating potential. Thevoltage sources V and V are set to zero or ground potential, and thecollector is connected through switch 53 to the voltage source VDrift-tube electrode and collector 10 are, therefore, directly connectedin undepressed collector operation. Biasted-output gap 22, operation forthe narrow-gap klystron for various values of cathode voltage V is shownas curves of FIGURE 2. It is seen that the available R.F. output powerto the load increases as the bias voltage V V across the output gap 22increases. It was found that the available output power is related tothe bias voltage V by the simple equation P :I (V +kV when P is theavailable output power I is a constant representing the fundamentalcomponent of the RF. beam current at the output gap 22, V is the cathode6 voltage, and k is a constant equal to unity for the narrow-gap klystonand greater than unity for the wide-gap klystron (curves 26 of FIGURE2). In both narrowand widegap tube operation, for biased-output gap 22operation (non-depressed collector 10) the DC. beam current I 8, isdetermined by the magnitude of the cathode voltage V and the power V 1is supplied by source V The beam 8 is intercepted at the other end ofthe klystron either on electrode 20 or collector 10 both of which areconnected to potential source V =V, which must, therefore, supply apower V l Thus, the DC. input power is given y D.C. 0( 0+ b)- For thenarrow-gap klystron where k=1, the R.F. output power P increases at thesame rate as the DC. input power P and hence biased output gap(non-depressed collector) operation does not increase efilciency.However, there is some advantage to biased output gap operation even inthis case. Reference to curve 25' of FIGURE 2 shows that the 2 kw. ratedoutput of this tube is obtained at V =8 kv. If it is assumed as is thecase in many high power klystrons, that the cathode voltage V is thelimiting factor on the maximum output power and that in this case the 8kv. cathode voltage is the limiting factor on output power, it is seenthat greater power than 2 kw. is obtained with V =8 kv. and output gapDC. voltage V =V 0 even though the efiiciency is not changed.

For the wide-gap klystron where k 1, reference to FIGURE 2, curves 26shows that the RF. power is seen to increase at a greater rate than theincrease in DC. input power from the biased gap 22 source and,therefore, the efliciency increases with bias voltage V :V even fornon-depressed collector.

Increased etficiency can be obtained with biased output gap 22 operationif collector 10 is connected through switch 53 to a potential V V Thistype of operation is termed depressed collector operation. FIGURE 3illustrates the efliciency improvement where the collector 10 isconnected to V ground potential and different values of voltage V areapplied to the electrode 20 for certain fixed values of cathodepotential V Curves 31 show that a significant increase in efliciency isobtained with depressed collector operation. Operation with V V where V=V is a larger fraction of V than the values of FIGURE 3, has been foundto result in significant increases in efficiency also. The increase inefficiency for depressed collector operation is attributed to the factthat the RF. power 'output into load 30 remains the same Whether or notthe collector 10 is depressed; however, the DC power input from source Vand V is reduced. The beam current I divides between electrode 20 andcollector 10; the DC. power input [V (aI +V (1x)I where a is the ratioof the current on electrode 20 to the current on collector 10, fordepressed collector operation is less than the DC. power input V I forundepressed collector operation. Optimum efliciency occurs for thatcombination of collector voltage V, and electrode 20 voltage V whichresults in minimum DC power input. It should be noted that although thecollector 10 of the experimental tube was not designed for depressedoperation, significant increase in efficiency was obtained. The highestefiiciency achieved was approximately 65%. However, the results shownare by no means optimum and higher efliciencies are expected for thesituation where the collector is designed for depressed operation.

It should be noted that in other techniques toincrease efficiency bydepressed collector operation, the kinetic energy of the spent team isrecovered as DC energy, and is often dissipated in a resistor andassumed to be recoverable as DC. energy at the cathode input supply VThis is not the case with the depressed collector biasedgap klystronopera-tion in accordance with this invention. Here the energy of theelectron beam 8 is converted directly as RF. energy, and the efiiciencycalculated is the overall electronic efliciency of the klystron with noattempt to recover D.C. energy at the collector for subsequentutilization as DC. input energy.

A biasing potential across the penultimate cavity gap 23 or across thepreceding buncher cavity gap 24 is also effective in increasing klystronefficiency and hence power output. Bias potential V V is applied acrossgap 23 for the condition V =0 and V =V =V The curve 41 of FIGURE 4 showsthe efiiciency for this condition of operation for V =5 kv. and 52%,20.A comparison of curve 41 with curve 42, which is the efiiciency curve ofthe experimental tube conventionally operated with no accelerating biaspotential, shows that the biased penultimate gap 23 produces asignificant increase in efficiency. For instance, for a total beamvoltage V +V =8 kilovolts, the efficiency of the klystron with thebiased penultimate cavity 14 is approximately 63% as compared to anefliciency of approximately 56% for the conventionally operated tube ata beam voltage V of 8 kv. The increase in the efi'iciency obtained bybiasing the penultimate cavity 14 is attributed to the effective changein electrical length :of the drift-tube 16 between the penultimatecavity 14 and output cavity 18, and secondarily to the increase in thefundamental component of the RF. current at the output gap 22. Thechange in electrical length of the drift tube 16, caused by the changein velocity of the electrons under the influence of the DC acceleratingpotential at gap 23 is thought to result in improved performance becauseof shifting of the bunched R.F. current maximum with respect to theoutput gap to a more optimum position. Inspection of curve 41 of FIGURE4 shows that there is a limited region of bias voltage V where a peak inefficiency occurs. It is also believed that the bunching of the electronbeam is increased by the biased gap whereby the fundamental component ofRF. in the beam current is increased.

The maximum efiiciency for undepressed collector 10 operation with thebiased penultimate cavity gap 23 is seen by inspection of curve 41 ofFIGURE 4 to be 63%. However, efficiency of greater than was achievedwith depressed collector operation when collector 10 was groundedthrough switch 53. The increased efliciency of the biased penultimategap 23 with depressed collector is believed attributable to the samecause advanced for biased output gap 22 depressed collector operation.It was found that the cathode 6 voltage V did not appreciably affect themaximum efliciency of biased penultimate :gap 23 operation althoughcurve 41 for V =5 kv. was slightly different for higher values ofcathode voltage V An accelerating potential V =V across the gap of thebuncher cavity 11 preceding the penultimate cavity 14 resulted in anefficiency curve 43 of FIGURE 4 which was intermediate the unbiasedklystron efiiciency curve 42 and the biased penultimate cavityefiiciency curve 41. Depressed collector operation with V.,:() andresulted in a maximum efficiency of about 60%.

It is to be concluded that depressed collector biased penultimate gap 23operation results in the optimum efficiency and power output. However,it is possible to apply accelerating potentials across other gaps,particularly the output gap 22, at the same time although theexperimental tube did not indicate that this was a superior mode ofoperation over the depressed collector-biased penultimate cavity gapalone.

The chokes 13, 15, 17, 19 and 21 are designed to provide an effectiveR.F. short circuit to electrical energy within their respective cavitiesat the input slot 32 of each choke. The location of the choke input slot32 is of pri mary importance and should be located at a region of itsassociated cavity where the R.F. current is a minimum. The location ofthe current minimum depends on the particular cavity configuration andfor most cavities used in practice must be determined experimentally. Ina practical situation, mechanical considerations may require that thechoke be placed other than this current minimum location. For example,in the applicants tube which uses external cavities, it was foundmechanically convenient and electrically satisfactory to place thechokes at the junction where the external cavities make electricalcontact with contact rings of the drift tubes.

The choke itself is a conventional three transmission line design suchas that described in Principles of Microwave Circuits, Montgomery, Dickeand Purcell, Mc- Graw-Hill, 1948, page 198, where the length andimpedance of lines 33, 34, and 35 are selected by the use oftransmission line formulas to produce a short circuit impedance at theinput slot 32 for a free space termination at output slot 36 over a widefrequency band. A three section choke properly designed and located inthe cavity was found to have very low level of R.F. energy radiationfrom the output slot 36.

Biased gap operation of a klystron provides other desirable featuresother than increased efficiency and R.F. power output. It is found thatthere is an increase in beam transmission and better high-gainstability. It is also expected that possibility of multipactor isreduced.

Multipactor is defined as the electronic gap admittance resulting from asustained secondary-emission discharge produced in a gap by the motionof the secondary electrons in synchronism with the electric field in thegap. Attempts by others to eliminate multipacting by coating one edge ofthe output gap 22 with a low secondary emission material results in alossy cavity and hence is undesirable. Also indenting the edges of thegap 22 to destroy the symmetry which is helpful to synchronism has beenmoderately successful. The biased output gap should minimize themultipactor problem since the D.C. potential across the gap destroys thesymmetry in the electric field in the gap since the field in onedirection is greater than in the other during one cycle of the R.F.voltage in the gap.

Interception of even a small fraction, less than 5%, of

the D.C. beam current I by an element of the klystron other than thecollector 10 or the portion of the tube electrode 20 facing collector10, is highly undesirable in a high power klystron. In general,interception is found to take place primarily at the edge of the outputgap 22 where the localized region of impact of the intercepted electronscauses severe heating where interception of only 2% of the beam 8 of a 1megawatt klystron resulting in 20 kilowatts of power to be dissipated.Biased gap operation of the experimental klystron caused the beam 8transmission from cathode 6 to collector It) to increase from 97% toover 99%. Therefore, it is seen that biased output gap or penultimategap operation will reduce the 10- calized heating problem due to beaminterception.

High gain stability of klystrons is also seen to be improved by biasedgap operation. In conventional klystrons, any electron which has beendecelerated to a point where its velocity direction has reversed willreturn to the gap of input cavity 1 without difficulty. Instability isthe likely result of the presence of such electrons. The presence of theD.C. biased gap provides an electric field which opposes the return ofthese reversed electrons to the input cavity 1 resulting in greaterisolation of the R.F. output and input and hence greater stability.

It was mentioned earlier that the application of an accelerating voltageto the cavity gaps 24 and 23 causes a change in the electrical length ofdrift tube sections 12 and 16 respectively. This change in electricallength may be used to phase control the output R.F. energy relative tothe input R.F. energy. This may be accomplished by tapping a smallportion of the R.F. output energy of load 30 into a phase detector whoseother input is the R.F. input energy source 5. The D.C. output from thisphase detector is D.C. amplified to control the voltage applied to drifttube sections 16 or 12 instead of using fixed potential sources V and VIt is also seen that since the accelerator voltage V V and V affect theoutput power, the output power amplitude can be controlled by anamplitude detector sampling the output power and then being connected toa controllable source of D.C. voltage which is used in place of any ofthe fixed potential sources V1, V2, 01' V3.

Although the tube described was operated as an amplifier, it is not tobe construed that this invention could be so restrictive. It is believedthat improved operation of a klystron functioning as an oscillator wouldalso be obtained using bias gap construction. Although a klystron hasbeen used as the preferred embodiment of this invention, the principleof bias gap acceleration of an electron beam would be applicable toother tubes in which velocity modulation of an electron beam is used inconjunction with drift tube sections.

What is claimed is:

1. An electron beam modulation device comprising, an electron beamsource for providing a beam having a defined axis, a plurality ofcavities of the reentrant type, said cavities having drift tube sectionsas their reentrant portions through which the beam passes, said drifttube sections of each cavity being in axial alignment and in spacedapart relation to form a gap, said gap being a part of said cavity,whereby an R.F. electric field within said cavity provides an R.F.electric potential across said gap to act on said beam, means forapplying a D.C. potential across at least one of said gaps to provide aD.C. electric field across the gap acting upon the beam to increase itsvelocity, and a collector for termination of said beam.

2. The apparatus as defined in claim 1 comprising in addition aplurality of R.F. chokes, at least one choke of said plurality beingplaced between the drift tube forming one side of the gap to which aD.C. potential is applied and the cavity connected to the drift tubeforming the other side of the gap, said choke providing an electricalshort circuit at the resonant frequency of the cavity while providing anopen circuit for the D.C. potential applied across the gap.

3. The apparatus as in claim 1 comprising in addition said collectorbeing connected to a source of D.C. potential to provide a deceleratingD.C. electric field acting on said beam to reduce its velocity beforeterminating on said collector.

4. The apparatus as defined in claim 1 comprising in addition at leastone R.F. choke placed between each drift tube forming one side of thegap to which a D.C. potential is applied and the said cavity, said chokeproviding an electrical short circuit at the resonant frequency of thecavity while providing an open circuit for the D.C. potential differencebetween said drift tube and the remainder of said cavity.

5. The apparatus as defined in claim 2 wherein said cavity has a regionof low R.F. current density at its resonant. frequency, said choke beinglocated in the region of low current density in the cavity.

6. An electron beam modulation device comprising, an electron beamsource for providing a beam having a defined axis, a plurality ofcavities each resonant at an R.F. frequency, each cavity having a gap toallow said beam to pass through the cavity whereby said beam is actedupon by the RP. field at each cavity gap to change its velocity andspace charge distribution, said plurality of cavities including an inputcavity, an output cavity and intermediate cavities, means for applying aDC. potential across the gap of the one of said intermediate cavitiesadjacent said output cavity, whereby said potential produces a DCelectric field in said cavity gap to cause said beam to increase invelocity, and a collector for terminating said beam.

8 7. The apparatus as in claim 6 comprising in addition, means forproviding a DC. decelerating electric field acting on said beam toreduce the velocity of the beam in the region between the output cavityand the collector.

References Cited by the Examiner HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner.

1. AN ELECTRON BEAM MODULATION DEVICE COMPRISING, AN ELECTRON BEAMSOURCE FOR PROVIDING A BEAM HAVING A DEFINED AXIS, A PLURALITY OFCAVITIES OF THE REENTRANT TYPE, SAID CAVITIES HAVING DRIFT TUBE SECTIONSAS THEIR REENTRANT PORTIONS THROUGH WHICH THE BEAM PASSES, SAID DRIFTTUBE SECTIONS OF EACH CAVITY BEING IN AXIAL ALIGNMENT AND IN SPACEDAPART RELATION TO FORM A GAP, SAID GAP BEING A PART OF SAID CAVITY,WHEREBY AN R.F. ELECTRIC FIELD WITHIN SAID CAV-